Microstrip impedance matching circuit with harmonic terminations

ABSTRACT

A microstrip impedance matching circuit has fixed harmonic terminations in the form of a pair of open-circuited stubs, each having a length equal to a quarter wavelength of a different harmonic frequency, connected to the main transmission line to cause the impedance at the harmonic frequencies to be made constant irrespective of the nature of the load impedance at the harmonic frequencies.

United States Patent Jacobs et al.

[151 3,662,294 [451 May 9,1972

[54] MICROSTRIP IMPEDANCE MATCHING CIRCUIT WITH HARMONIC TERMINATIONS[72] lnventors: Paul H. Jacobs; Robert L. Peay, both of Schaumburg;David S. Wisherd, Mt. Prospect, all of ill.

[73] Assignee: Motorola, Inc., Franklin Park, Ill.

[22] Filed: May 5, 1970 [21] Appl. No.: 34,695

[52] US. Cl ..333/33, 333/73, 333/84 M, 321/69 [51] Int. Cl ..H03b 7/38,HOlp 3/08 [58] Field ofSearch ..333/84 M, 73 W, 33, 73 S; 321/69 [56]References Cited UNITED STATES PATENTS 3,343,069 9/ 1967 Tsuda ..321/693,402,340 9/ 1 968 Ringereide. ...32 l /69 3,345,589 10/1967 DiPiazza... ...333/73 3,309,626 3/1967 Higgins ...333/l7 3,292,075 12/1966Wenzel ..321/69 Primary Examiner-Herman Karl Saalbach AssistantExaminer-C. Baraff Attorney-Mueller & Aichele [5 7] ABSTRACT Amicrostrip impedance matching circuit has fixed harmonic terminations inthe form of a pair of open-circuited stubs, each having a length equalto a quarter wavelength of a different harmonic frequency, connected tothe main transmission line to cause the impedance at the harmonicfrequencies to be made constant irrespective of the nature of the loadimpedance at the harmonic frequencies.

3 Claims, 3 Drawing Figures MICROS'IRIP IMPEDANCE MATCHING CIRCUIT WITHI-IARMONIC TERMINATIONS BACKGROUND OF THE INVENTION Strip linetransmission systems or microstrip circuits are commonly used inmicrowave transmitters and receivers. One use of the microstrip circuitsis to provide an impedance matching circuit between the output of anamplifier stage and a load, which may be the input circuit of anotheramplifier or a band-pass or low-pass filter or the like. Microstripimpedance matching circuits are designed to transform the load impedanceto the complex impedance at the fundamental frequency necessary forproper operation of the system. Such devices work satisfactorily so longas the load is a constant pure resistance at all frequencies.

Certain loads, however, present the correct impedance at the fundamentalfrequency but present reactive terminations at harmonic frequencies.Since RF devices are sensitive to harmonic terminations, the poweroutput and band width of the amplifier can be greatly affected. If theload is mismatched at harmonic frequencies, the power output of theamplifier is low and the response is skewed. It appears that thechanging load at the harmonic frequencies shifts the operating point ofthe amplifier and affects its gain at the fundamental frequency. As aconsequence, it is desirable to cause the impedance at the harmonicfrequencies to be made constant.

SUMMARY OF THE INVENTION Accordingly, it is an object of this inventionto provide an improved microstrip impedance matching circuit.

It is a further object of this invention to provide harmonicterminations on a microstrip impedance matching circuit, causing theimpedance at harmonic frequencies to be made constant.

It is an additional object of this invention to attach stubs to the maintransmission line of an impedance matching microstrip circuit, with thelengths of the stubs being selected to reflect short circuits to theharmonic frequencies at the points where the stubs are attached to themain transmission line.

In accordance with a preferred embodiment of this invention, amicrostrip impedance matching circuit for coupling an RF amplifier to aload includes a conductive strip having harmonic termination circuitmeans coupled to the conductive strip for providing a short circuit tosignals of at least one harmonic frequency on the conductive strip. Morespecifically, the harmonic termination is in the form of a quarterwavelength open-circuited stub at the harmonic frequency, with thelocation of the stub on the conductive strip of the impedance matchingnetwork being such as to present substantially an open circuit at theharmonic frequency to the input of the matching circuit and to present ashort circuit to reflected signals at the same frequency.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a block diagram of a radiotransmitter with which the impedance matching circuit of a'preferredembodiment of this invention may be used;

FIG. 2 is a detailed circuit diagram of an impedance matching circuit inaccordance with a preferred embodiment of the invention; and

FIG. 3 is a perspective view of a' microstrip impedance matching circuitof the type shown in FIG. 2.

DETAILED DESCRIPTION Referring now to FIG. 1, there is shown an RFtransmitter circuit of the type used in a mobile communications unit orthe like. The basic operating frequency for the transmitter shown inFIG. 1 is provided by a 30 MHz oscillator which supplies signals at l mwto a broad band modulator circuit 11. Audio input signals to bemodulated are obtained from a conventional microphone l2 and are passedthrough an audio amplifier stage 13 to the modulator l 1, the output ofwhich also is a 30 MHz signal supplied to a broadband multiplier circuit15 providing a 450 MHz signal at l mw. The signals present at the outputof the multiplier 15 are passed through a band-pass filter 17, which maybe'in the form of a microstrip filter circuit if so desired, and theoutput of the filter 17 is a 450 MHz signal at 0.5 mw.

In order to provide a signal at sufficient power to the antenna of thetransmitter, a microstrip amplifier circuit 18 responds to the outputsignals from the filter 17 to provide a 450 MHz output signal at 50watts. This signal is passed through a harmonic filter 19, whichpreferably is in the form to provide an impedance matching circuit formatching the 7 output of the transistor amplifier normally used to theinput of the harmonic filter 19. The impedance matching circuit musttransform the load impedance to the complex impedance at the fundamentalfrequency(450 MI Iz) necessary for proper operation of the system.

Referring now to FIG. 2, there is shown an impedance matching networkutilizing a microstrip transmission line in accordance with a preferredembodiment of this invention. This network may be included as part ofthe microstrip amplifier circuit 18 as indicated in dotted lines in FIG.2. The output of the impedance matching network then is supplied to asuitable load, such as the harmonic filter 19 or someother load such asadditional amplification stages, and in FIG. 2. this load is shown as asimple resistor enclosed in dotted lines and identified as load 23. p

The output transistor of the amplifying stage which is to supply inputsignals to the input end of the impedance matching network is shown asan NPN transistor 24, with input signals such as would be obtained fromearlier amplification stages or from the band-pass filter 17 beingapplied to the base of transistor 24. The collector of the transistor 24is connected to a main impedance matching transmissionline or conductivestrip 25, the other end of which is connected to the load 23. Themicrostrip or strip line circuit is formed by attaching the conductivestrip 25 to one side of a sheet of dielectric material 30, with theother side of the sheet'of dielectric material 30 being covered with aground plane in the form of a conductive coating 31, best shown in FIG.3.

To match the impedance of the amplifier represented by transistor 24 tothat of the load 23, an impedance matching stub 26 is connected to theconductive strip 25 at the end near the load 23. The dimensions of thestub 26 and the length of the strip 25 (for a given characteristicimpedance) may be' chosen in accordance with well-known techniques totransform the load impedance to the desired complex impedance at thefundamental frequency. So long as the load is a constant pure resistanceat all frequencies, the conductive strip 25 and the stub 26 would besufficient to provide a satisfactory impedance match.

When a load such as the harmonic filter 19, however, is used, the loadpresents reactive terminations to signals at harmonic frequencies sincethe filter 19, while exhibiting a pure impedance at a fundamentalfrequency, is mismatched at all harmonics of that fundamental frequency.As a consequence, the power output or apparent gain of the amplifier islow and the frequency response characteristics are changed. In order toprevent the impedance matching circuit from being adversely affected bythe harmonics of the fundamental operating frequency of the amplifier,open-circuited quarter wavelength conductive stubs 27 and 28 areattached at right angles to the main conductive strip 25 and operate asharmonic terminations or short circuits at different harmonicfrequencies. Thus, irrespective of the impedance which is connected atthe output or right-hand end of. the matching circuit, the harmonicimpedance presented to the input or left-hand end is constant.

The harmonic frequencies which constitute the greatest problem are thesecond and third harmonics, so that the stub 27 is selected to have alength l, equal to one-fourth wavelength of the third harmonic frequencyand the stub 28 is selected to have'a length l equal to one-fourthwavelength of the second harmonic frequency of the fundamental frequency(900 MHz in the example shown in FIG. 1). The stubs 27 and 28 then placea short circuit at these respective harmonic frequencies on the seriesmatching line 25 at the points where the stubs are attached to the line25, and the position of these short circuit points for the harmonicfrequencies may be adjusted to present any desired reactive impedance tothe input at the harmonic frequencies. As a result any impedance that isreflected from the load is connected in parallel with these shortcircuits, so that changes in the load termination at harmonicfrequencies cannot affect the input impedance of the matching circuit.Thus, a constant impedance at the harmonic frequencies is presented tothe signals on the collector of the transistor amplifier 24.

To provide optimum performance of the transistor amplifier 24, the inputimpedance at the left end of the conductive strip 25 shown in FIGS. 2and 3 should match the output impedance of the transistor. For purposesof illustration assume that this impedance is 8.9 j 9.5 ohms at thefundamental frequency. This is a typical value which maybe encounteredin applications of the circuit. At the same time the input impedance foroptimum performance should beinfinite at both the second and thirdharmonic frequencies. in the construction of the impedance matchingcircuit, the stubs 27 and 28 may be of any desired impedance, and forthe purpose of illustration assume that characteristic impedances of thelengths l and 1 are each equal to 30 ohms.

in order to cause the input impedance at the third harmonic to beinfinite or an open circuit impedance at the input to the stripconductor line 25, the length l must be made equal to the length 1,,namely a quarter wavelength at the frequency of the third harmonic or aone-twelfth wavelength at the fundamental of the input signal. Forconvenience, the characteristic impedance of the length l, also may be30 ohms.

To provide an open circuit (infinite impedance) for the second hannonicfrequency at the input to the left end of the impedance matching circuitconductive strip 25 at the collector of the transistor 24, it isnecessary for the point of connection of the stub 28 to the strip 25 toappear as if it were a quarter wavelength away at the second harmonicfrequency from the collector of the transistor 24. If the stub 27 werenot present, this could be accomplished merely by making the combinedlengths l, and i equal to the length l which is a quarter wavelength ofthe second harmonic frequency. The presence of the stub 27, however,must be taken into account since this impedance of this stub isconnected in parallel with the impedance of the length i presented tothe collector of the transistor 24 at the input end of the conductivestrip 25.

From a Smith chart, copyright 1949 by Kay Electric Company, theimpedance at the mid-point of the junction of the stub 27 with the mainconductive strip 25 must be +j 0.575 (30) or +j l7.2 ohms, and theimpedance looking into the length l, at the junction point of the stub27 with the main conductive strip 25 is -j 17.2 ohms.

The impedance of the length I, then must be chosen so that whenit isplaced in parallel with the impedance j 17.2, it results in a netimpedance of +j 17.2, which is the condition providing an open circuitfor the second harmonic at the input to the left end of the strip 25.Calculation of this impedance then shows thatthis parallel impedancemust amount to +j 8.6 ohms. If the characteristic impedance of thelength l is chosen to be 30 ohms, the short presented by the secondharmonic stub 28 at the junction of the mid-point of this stub with themain strip line 25 must be transformed to +1 8.6 ohms on a 30 ohm lineor 8.6/30 0.286. As a consequence the length of 1 as determined from aSmith chart is equal to 0.044 wavelengths at the second harmonicfrequency or 0.022 wavelengths at the fundamental frequency. As statedpreviously, the length I, is one-fourth the wavelength at the secondharmonic frequency, or one-eighth the wavelength of the fundamentalfrequency, in order to provide a short circuit stub at.

the second harmonic frequency.

The remaining dimensions 1,, I, and l, indicated in FIGS. 2 and 3 thenmust be chosen to provide the desired'transformation of the inputimpedance (8.9 +j 9.5 ohms) to a 50 ohm load at the fundamentalfrequency. To determine the relative values of the impedances in thebranch 1 and the branches 1,, 1,, 1, along with the lengths of thesebranches, it is necessary to start at this amount (8.9 +j 9.5) and move0.0833 wavelength of the fundamental frequency (one-quarter thewavelength of the third harmonic frequency) toward the load. The loadthen is 30 (0.28 -j 0.19) or 8.4 -j 5.7 ohms. The load presented by thelength l, ofthe stub 27 is j 1.74) 30 =-j 52.2 ohms. The parallelcombination of this impedance and the load presented by the length 1:,between the midpoints of the stubs 27 and 28 must be equal to 8.4 j 5 .7ohms. Thus, to determine the load presented by the length l the solutionis as follows:

Where 2,. parallel combined load, and Z2 load presented by 1 Thus,-

Normalizing this to the 30 ohm line, as discussed previously,- provides(10.3 j 4.55 )/30 0.344 j 0.152 moving 0.022 wavelength at thefundamental frequency C(l length on a 30 ohm line gives:

(0.365 j0.275) 3O :11 -j 8.25

As a consequence, the parallel combination of the length l, of the stub28 and the remainder of the circuit to the right of the junction of themidpoint of the stub 28 with the main conductive strip 25, whichincludes the lengths l and I,, along with the length 1 of theconventional matching stub 26, must present an impedance of l l-j 8.25.Although the length 1 is fixed at a quarter wavelength of the secondharmonic frequen cy, the impedance of this length may be arbitrarilyselected, with the resulting remaining impedance for the parallelcombination being provided by adjustment of the relative dimensions 1 land l, to obtain the desired matching impedance to the 50 ohm load 23.These adjustments, however, have no affect on the open circuitconditions presented to the input at the-leftend of the strip line 25for the second and third harmonic'frequencies.

The'microstrip impedance matching circuit described may be utilized tocouple cascaded amplifier stages or may be used to couple an amplifierstage to a filter stage in the manner described. The stubs 27 and 28 maybe placed at other positions on the line 25, .if infinite inputimpedance to these frequencies is not desired. The exact position of thestubs may 7 be adjusted for optimum system performance, and locating thestubs to present a substantially open circuit condition to signals atthe harmonic frequencies applied to the input of the impedance matchingdevice is considered to provide such optimum performance. It has beenobserved that harmonic short circuits provided by the open-circuitedstubs 27 and 28 results I in a 5 to 10 percent increase in power' outputover conven- ;tional L-C matching circuits, which present a highimpedance at the harmonic frequency.

Although the foregoing description has been limited to harmonicimpedances in the form .of open-circuited quarter wavelength stubs toprovide the short circuits at the harmonic frequencies, it is apparentthat RF grounded stubs of half wavelengths at the harmonic frequenciesalso could be used to produce the same results. The quarter wavelengthstubs are preferable, permitting a more compact microstrip layout. Inaddition, the open-circuited quarter wavelength stubs do not require anyadditional circuit components. If half wavelength stubs are because thelength of the stubs is shorter, thereby used, it is necessary toterminate each of the stubs to ground through an additional capacitor.This results in increased cost and additional assembly operations.Furthermore, the operation of the impedance matching circuit over theband of frequencies immediately adjacent the fundamental frequency isbetter if the short circuit terminating stubs are kept as short aspossible, since the shorter stubs are less frequency sensitive thanlonger stubs of the type which would be necessitated if the halfwavelength stubs were employed.

in the foregoing description, the specific example considered amicrostrip line having a characteristic impedance of 30 ohms, which is aconventional line width commonly employed. Different characteristicimpedances, however, may be utilized if desired. The impedance matchingof the circuit at the fundamental frequency then would be dependent onthe characteristic impedance, on the length of the stub 26, and on theimpedance presented at the fundamental frequency by the short circuitingstubs 27 and 28. The characteristic impedance, however, of the stubs 27and 28 is of no significance at the harmonic frequencies since thelengths of the stubs are chosen to present short circuits to thesefrequencies at the points of attachment of the stubs to the conductivestrip 25.

An additional conservation of the physical dimensions of the impedancematching circuit, including the harmonic termination stubs 27 and 28, iseffected by placing the third harmonic terminating stub 27 nearest theinput end of the conductive strip 25, since a shorter stub is requiredto terminate the third harmonic at the collector of the transistor 24,the length l must be equal to the length 1 Thus, if the stub 27 is ashort circuit for the third harmonic frequency, the length 1 is shorterthan if the stub 27 were to provide a short circuit at the secondharmonic frequency. If such a conservation in the dimensions of theimpedance matching circuit is not desired, the stubs l and 1 shown inFIGS. 2 and 3 could be interchanged, with the length 1 then beingincreased to equal the length 1 Because the particular characteristicsof the microstrip circuitry can be relatively accurately controlled inthe manufacturing process consistent reproduction of the device ispossible. The predictable characteristics of the microstrip circuitrydescribed above are considered superior to techniques utilizing lumpedcircuit constants since control of lumped circuit constants is moredifiicult to implement.

We claim: 1. A microstrip impedance matching circuit for coupling an RFamplifier, operating at a fundamental frequency, to a load including incombination:

a sheet of dielectric material; a conductive coating on one side of saidsheet of dielectric material;

impedance matching circuit means including a conductive strip having aninput end and an output end on the other side of said sheet ofdielectric material for coupling an amplifier to a load; and

first and second open-circuited stubs connected to and extendingsubstantially from said conductive strip, the length of said first stubbeing selected to be one-fourth the wavelength of the second harmonic ofthe fundamental frequency and the length of the second stub beingselected to be one-fourth the wavelength of the third harmonic of thefundamental frequency, said first and second stubs being connected tosaid conductive strip at locations presenting substantially opencircuits at said second and third harmonic frequencies to signalsapplied to the input end of said conductive strip, with said first stubreflecting a short circuit to signals at said second harmonic of thefundamental frequency at the point where said first stub is connected tosaid conductive strip, and said second stub reflecting a short circuitto signals at said third harmonic of said fundamental frequency at thepoint where said second stub is connected to said conductive strip.

2. The combination according to claim 1 wherein the one of said firstand second open-circuited stubs which is connected to said conductivestrip nearest the input end thereof is spaced from said input end by adistance equal to the length of such stub.

3. The combination according to claim 2 wherein said secondopen-circuited stub is located nearer the input end of said conductivestrip than said first open-circuited stub.

1. A microstrip impedance matching circuit for coupling an RF amplifier,operating at a fundamental frequency, to a load including incombination: a sheet of dielectric material; a conductive coating on oneside of said sheet of dielectric material; impedance matching circuitmeans including a conductive strip having an input end and an output endon the other side of said sheet of dielectric material for coupling anamplifier to a load; and first and second open-circuited stubs connectedto and extending substantially 90* from said conductive strip, thelength of said first stub being selected to be one-fourth the wavelengthof the second harmonic of the fundamental frequency and the length ofthe second stub being selected to be one-fourth the wavelength of thethird harmonic of the fundamental frequency, said first and second stubsbeing connected to said conductive strip at locations presentingsubstantially open circuits at said second and third harmonicfrequencies to signals applied to the input end of said conductivestrip, with said first stub reflecting a short circuit to signals atsaid second harmonic of the fundamental frequency at the point wheresaid first stub is connected to said conductive strip, and said secondstub reflecting a short circuit to signals at said third harmonic ofsaid fundamental frequency at the point where said second stub isconnected to said conductive strip.
 2. The combination according toclaim 1 wherein the one of said first and second open-circuited stubswhich is connected to said conductive strip nearest the input endthereof is spaced from said input end by a distance equal to the lengthof such stub.
 3. The combination according to claim 2 wherein saidsecond open-circuited stub is located nearer the input end of saidconductive strip than said first open-circuited stub.